Long Term Evolution (LTE), as the next evolutionary technology of the 3rd generation mobile communication system known as Universal Mobile Telecommunication Service (UMTS), is designed to provide improved high speed packet data services based on the Orthogonal Frequency Division Multiplexing (OFDM).
FIG. 1 illustrates a diagram for an LTE system to which the radio link failure detection method of the present invention is adopted.
As shown in FIG. 1, an LTE system is characterized with the Evolved Radio Access Network (hereinafter called E-RAN) 110 and 112 including two infrastructure nodes: the Evolved Node B (hereinafter called ENB or Node B) 120, 122, 124, 126, and 128 and the anchor node 130 and 132. A User Equipment (UE) 101 connects to the Internet Protocol (IP) network via an eNB and an anchor node. The eNB is connected to the eNB through a radio channel and responsible for cell and radio resource management. For instance, the eNB broadcasts the control information in the form of system information within the cell, allocates radio resources to the UEs for transmission and reception of data and control information, and determines handover of the UEs based on the channel management information of the serving and neighbor cells. The eNB includes control protocols such as Radio Resource Control (RRC) protocol related to the radio resource management.
FIG. 2 illustrates a timing diagram for a Discontinuous Reception (DRX) mode operation of a UE in the LTE system of FIG. 1. The UE starts reception of Physical Downlink Control Channel (PDCCH) carrying the uplink/downlink scheduling information at the beginning of every DRX cycle and then communicates the data/control information through Downlink Shared Channel (DSCH) and Uplink Shared Channel (USCH) based on the scheduling information until a UE's DRX timer expires. The timer restarts whenever the scheduling information destined to the UE is transmitted on the PDCCH. A DRX cycle length is divided into an active time during which the UE receives the uplink/downlink scheduling information on the PDCCH and communicating the data/control information through the DSCH and USCH until the time expires and an inactive time during which the UE switches off the uplink/downlink channels to save energy.
The durations of the active time and inactive time vary depending on the operation of the DRX timer (in FIG. 2, the active time of the first DRX cycle is shorter than that of the second DRX cycle).
Meanwhile, the radio link between the UE and eNB can fail due to various causes, which is referred to as a Radio Link Failure (RFL). Additionally an RLF detection mechanism is required to be defined in the LTE system. Typically, the RLF detection is performed such that the UE monitors the radio channel status to detect disconnection to the base station. However, no clear RLF detection mechanism for the UE operating in DRX mode as shown in FIG. 2 is specified in 3rd Generation Partnership Project (3GPP) standards. There is therefore a need of an RLF detection method for the UE operating in DRX mode.